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Aluminium silicate

Aluminum silicate (or aluminium silicate) is a name commonly applied to chemical compounds which are derived from aluminium oxide, Al<sub>2</sub>O<sub>3</sub> and silicon dioxide, SiO<sub>2</sub> which may be anhydrous or hydrated, naturally occurring as minerals or synthetic. Their chemical formulae are often expressed as xAl<sub>2</sub>O<sub>3</sub>·ySiO<sub>2</sub>·zH<sub>2</sub>O. It is known as E number E559.

Main representatives

Andalusite, kyanite, and sillimanite are the principal aluminium silicate minerals. The triple point of the three polymorphs is located at a temperature of and a pressure of . These three minerals are commonly used as index minerals in metamorphic rocks.

  • Al<sub>2</sub>SiO<sub>5</sub>, (Al<sub>2</sub>O<sub>3</sub>·SiO<sub>2</sub>), which occurs naturally as the minerals andalusite, kyanite and sillimanite which have distinct crystal structures.
  • Al<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>, (Al<sub>2</sub>O<sub>3</sub>·2SiO<sub>2</sub>), called metakaolinite, formed from kaolin by heating at .
  • Al<sub>6</sub>Si<sub>2</sub>O<sub>13</sub>, (3Al<sub>2</sub>O<sub>3</sub>·2SiO<sub>2</sub>), the mineral mullite, the only thermodynamically stable intermediate phase in the Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> system at atmospheric pressure. This also called '3:2 mullite' to distinguish it from 2Al<sub>2</sub>O<sub>3</sub>·SiO<sub>2</sub>, Al<sub>4</sub>SiO<sub>8</sub> '2:1 mullite'.
  • 2Al<sub>2</sub>O<sub>3</sub>·SiO<sub>2</sub>, Al<sub>4</sub>SiO<sub>8</sub> '2:1 mullite'.

The above list mentions ternary materials (Si-Al-O). Kaolinite is a quaternary material (Si-Al-O-H). Also called aluminium silicate dihydrate, kaolinite occurs naturally as a mineral. Its formula is Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>, (Al<sub>2</sub>O<sub>3</sub>·2SiO<sub>2</sub>·2H<sub>2</sub>O).

Aluminium silicate composite materials, fibres

Aluminium silicate is a type of fibrous material made of aluminium oxide and silicon dioxide, (such materials are also called aluminosilicate fibres). These are glassy solid solutions rather than chemical compounds. The compositions are often described in terms of % weight of alumina, Al<sub>2</sub>O<sub>3</sub> and silica, SiO<sub>2</sub>. Temperature resistance increases as the % alumina increases. These fibrous materials can be encountered as loose wool, blanket, felt, paper or boards.

References

External links